In order to protect integrated circuits against voltages which can damage or destroy the integrated circuit, protective circuits are connected to the terminals (referred to as pads hereinafter) of the integrated circuit. In particular voltages (referred to as ESD voltages hereinafter) at the pads which are caused by electrostatic discharges (ESD=electrostatic discharge) can destroy sensitive structures on the integrated circuits.
A simple protective circuit often used for limiting high voltages at the pads comprises a resistor for input current limiting and two diodes, which are reverse-biased between the pad and a supply voltage and, respectively, a reference-earth potential of the integrated circuit. If a voltage at a pad exceeds the supply voltage, the diode reverse-biased between the pad and the supply voltage begins to conduct and the voltage at the pad is limited to the supply voltage. As a result, the control range of a voltage at a pad is limited to a range between the supply voltage and the reference-earth potential. The second diode begins to conduct if a voltage at the pad falls below the reference-earth potential.
It is often the case, however, that, by way of example, analogue circuit components on the integrated circuit require higher voltages than the supply voltage. In this case, however, the simple protective circuit is a hindrance since voltages at the pads which are larger than the supply voltage of the integrated circuit are limited to the supply voltage by the protective circuit.
U.S. Pat. No. 5,400,202, discloses a protective circuit having a thyristor (SCR=Silicon Controlled Rectifier), the thyristor triggering above a specific voltage at the pad and producing a low-impedance connection between the pad and a reference-earth potential of the integrated circuit, as a result of which ESD voltages at the pad are effectively limited. Thyristors can advantageously conduct high currents and can be turned on rapidly. In U.S. Pat. No. 5,781,388, a voltage divider is provided in a protective circuit, which voltage divider comprises a capacitance and a resistor and, in the event of voltage changes at the pad, generates a threshold voltage which is used for triggering a thyristor. These protective circuits have the disadvantage of the high triggering voltages required for turning on the thyristor.
The technical problem on which the invention is based consists, therefore, in specifying a protective circuit which avoids the disadvantages mentioned in the introduction.
The invention relates to a protective circuit connected to a pad of an integrated circuit. The protective circuit has a threshold detector device, which derives a threshold voltage from a voltage at the pad. The threshold voltage controls a first transistor, whose load path is connected to the pad at a first terminal and whose load path is connected to the control terminal of a second transistor at a second terminal. The load path of the second transistor is connected in parallel between the pad and a reference-earth potential and connects the pad to the reference-earth potential. It is advantageous that only a low threshold voltage is required for turning on the protective structure, the threshold voltage corresponding to just twice the turn-on voltage of a transistor.
In a particularly preferred embodiment, a multiplicity of transistors are provided instead of the second transistor, the control terminals of the multiplicity of transistors being connected to the second terminal of the load path of the first transistor and the load paths of the multiplicity of transistors being connected in parallel between the path and the reference-earth potential.
A particularly preferred embodiment relates to the threshold detector device, the latter having a voltage divider connected between the pad and the reference-earth potential. The threshold voltage can be tapped off at a node of the voltage divider. The voltage divider is advantageously constructed from high-value resistors which can be realized in a simple manner in integrated circuits in order that a current through the voltage divider is kept low.
In a particularly preferred embodiment, the transistors are npn bipolar transistors.
In a further preferred embodiment, a respective resistor is connected between the pad and the load paths of the transistors, the resistors serving for current limiting purposes.
In one embodiment, a diode is connected between the resistors and the pad. The diode turns off if a voltage at the pad falls below the voltage at the n terminal of the diode.
In a further embodiment, a resistor is connected between the second terminal of the load path of the first transistor and the reference-earth potential, which resistor serves for setting the operating point of the first transistor.